This invention relates to a pressure sensitive adhesive which is prepared by emulsion polymerizing an ethylenically unsaturated monomer and 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer. The alkoxy silane crosslinks during film formation to provide the adhesive with increased shear strength without the disadvantage of decreased loop tack.
An important characteristic of pressure sensitive adhesives is the shear strength, also known as the cohesive strength or the internal strength of the adhesive, which measures the tendency of a pressure sensitive adhesive to flow or creep under an applied load. Especially in rolled paper applications where the pressure sensitive adhesives are used at temperatures considerably above their glass-transition temperatures, the shear strength must be greater than the adhesiveness to prevent the adhesive from creeping beyond the edges of the paper and bleeding through the paper which results in sheets of paper sticking together.
A crosslinked pressure sensitive adhesive sample or one with a high degree of entanglement (higher viscosity) would be expected to have a higher resistance to shear than a pressure sensitive adhesive sample with few entanglements (low viscosity). For these reasons, silane monomers have been used to provide crosslinking in polymer compositions. U.S. Pat. No. 3,729,438 describes an aqueous dispersion of a crosslinkable latex polymer prepared from vinyl acetate and a vinyl hydrolyzable silane. Upon removal of water from the latex polymer, the polymer further polymerizes or cures into a crosslinked polymer.
U.S. Pat. No. 3,706,697 describes an emulsion polymerization product of an acryloxyalkylsilane monomer with an alkyl acrylic ester wherein the silane is introduced to the polymerization after a portion of the other monomers are polymerized. International Patent Application WO 97/15624 describes curable aqueous compositions prepared with an emulsifiable polymer having pendant and/or terminal silyl ester groups and pendant terminal acidic groups neutralized with fugitive bases.
The disadvantages of using such alkoxy silane monomers, however, are that (1) the alkoxy silane monomers may crosslink during the emulsion polymerization and form grit; (2) the alkoxy silane monomers may hydrolyze during the emulsion polymerization so that even if the alkoxy silane monomer was polymerized the reactivity necessary for subsequent cross-linking would be destroyed; or (3) the alkoxy silane modified latex may hydrolyze during storage.
There continues to be a need for pressure sensitive adhesive compositions which provide the necessary shear strength at high temperature and high relative humidity environments, and provide increased shelf life without the disadvantages present in prior art pressure sensitive adhesives.
Accordingly, it is an object of the present invention to provide a pressure sensitive adhesive composition.
It is also an object of the invention to provide a pressure sensitive adhesive which bonds to a substrate and maintains sufficient shear strength.
It is another object of the invention to provide a pressure sensitive adhesive which may be applied to paper without bleeding through the paper or exhibiting excessive creep beyond the edges of the paper.
With regard to the foregoing and other objects, the present invention provides a pressure sensitive adhesive composition comprising the emulsion polymerization product of at least one ethylenically unsaturated monomer and from 0.01 to 2 parts per hundred monomer (pphm), preferably from about 0.05 to about 1 pphm of a sterically hindered alkoxylated silane monomer, wherein the sterically hindered alkoxylated silane monomer has the structure
R1xe2x80x94Sixe2x80x94(OR2)n(R3)3xe2x88x92n
wherein R1 is selected from the group consisting of an alkylene, arylene, and aralkylene group; R2 is independently a sterically hindered alkyl group having 3 to 10 carbon atoms in a straight or branched chain configuration; R3 is a monovalent alkyl group having 1 to 10 carbon atoms; and n is an integer of from 1 to 3.
According to another aspect the invention provides a method of making a pressure sensitive adhesive, said method comprising (I) mixing an acidic metal salt curing agent with an aqueous emulsion polymer to form an adhesive, said polymer comprising the reaction product of at least one ethylenically unsaturated monomer and from 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer as described above; (II) applying the adhesive formed in Step (I) to a substrate; and (III) curing the adhesive with air or radio frequency to form a film.
An additional aspect of the invention provides a tape comprising a backing coated with the pressure sensitive adhesive composition as described above.
During film formation of the pressure sensitive adhesive of the invention, the sterically hindered alkoxysilane functionality of the silanes crosslink resulting in pressure sensitive adhesives which exhibit a significant increase in shear strength without the expected decrease in loop tack as compared to pressure sensitive adhesives prepared without sterically hindered alkoxylated silanes. This increase in shear strength allows pressure sensitive adhesives of the invention to be applied to paper under high relative humidity conditions (80% to 90%) wherein the paper is rolled without the adhesive creeping out or bleeding though the paper. Moreover, the presence of the sterically hindered alkoxy groups in the polymers minimizes crosslinking during emulsion polymerization and storage of the pressure sensitive adhesive.
The pressure sensitive adhesive compositions of the present invention are prepared from an aqueous emulsion polymer which is curable to form a film. The polymer is the reaction product of at least one ethylenically unsaturated monomer and a sterically hindered alkoxylated silane monomer. The sterically hindered alkoxylated silane is incorporated in the backbone of the polymer. The steric hindrance of the alkoxylated silane minimizes hydrolysis of the alkoxylated group during polymerization and storage. The aqueous emulsion polymer has acidic groups which are neutralized with a base. The release of the base upon application of the polymer frees the acid of the ethylenically unsaturated monomer for catalyzing the cure of the polymer.
While not wishing to be bound by any particular theory, the present inventors believe that crosslinking occurs between the sterically hindered alkoxysilane functionality on the polymer by means of a hydrolysis reaction to give silanols with subsequent condensation reaction between silanols and/or carboxyl groups on the polymer. Such crosslinking occurs during film formation of the pressure sensitive adhesive. The advantage of preparing the pressure sensitive adhesive with sterically hindered alkoxylated silane monomers is that crosslinking during the emulsion polymerization of the polymer and storage of the pressure sensitive adhesive, especially in the presence of carboxyl groups, is minimized.
The sterically hindered alkoxylated silane monomer is present in a critical amount of from 0.01 to 2 pphm, preferably 0.05 to 1 pphm. The sterically hindered alkoxylated silane monomer has the structure:
R1xe2x80x94Sixe2x80x94(OR2)n(R3)3xe2x88x92n
wherein R1 is selected from the group consisting of an alkylene, arylene, and aralkylene group; R2 is independently a sterically hindered alkyl group having 3 to 10 carbon atoms in a straight or branched chain configuration; R3 is a monovalent alkyl group having 1 to 10 carbon atoms; and n is an integer of from 1 to 3.
Suitable sterically hindered alkoxylated silane monomers for use in the invention include vinyltriisopropoxy silane, vinylpropyltriisopropoxy silane, vinylpropyltriisobutoxy silane, vinyltriisobutoxy silane, vinylpentyltri-t-butoxy silane, vinylbutyldimethyloctyl silane, vinylpropylmethyldipentoxy silane, vinylpropyltri-sec-butoxysilane. The sterically hindered alkoxylated silane monomer is preferably vinyltriisopropoxysilane.
Suitable ethylenically unsaturated monomers which are used to prepare the aqueous emulsion polymer include anhydrides, vinyl esters, alpha-olefins, alkyl esters of acrylic and methacrylic acid, substituted or unsubstituted mono and dialkyl esters of unsaturated dicarboxylic acids, vinyl aromatics, unsubstituted or substituted acrylamides, cyclic monomers, monomers containing alkoxylated side chains, sulfonated monomers, and vinyl amide monomers. A combination of ethylenically unsaturated monomers may also be used. As used herein, xe2x80x9cethylenically unsaturated monomerxe2x80x9d does not include ionic monomers.
Suitable anhydride monomers are, for example, maleic anhydride and itaconic anhydride. Suitable vinyl esters are, for example, vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl valerate, vinyl 2-ethyl-hexanoate, vinyl isooctanoate, vinyl nonanoate, vinyl decanoate, vinyl pivalate, and vinyl versatate. Suitable alkyl esters of acrylic and methacrylic acid are, for example, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, butyl acrylate, pentyl acrylate, hexyl acrylate, and 2-ethyl hexyl acrylate, etc. Suitable substituted or unsubstituted mono and dialkyl esters of unsaturated dicarboxylic acids are, for example, substituted and unsubstituted mono and dibutyl, mono and diethyl maleate esters as well as the corresponding fumarates. Suitable vinyl aromatic monomers preferably contain from 8 to 20 carbon atoms, most preferably from 8 to 14 carbon atoms. Examples of vinyl aromatic monomers are styrene, 1-vinyl naphthalene, 2-vinyl naphthalene, 3-methyl styrene, 4-propyl styrene, t-butyl styrene, 4-cyclohexyl styrene, 4-dodecyl styrene, 2-ethyl-4-benzyl styrene, 4-(phenylbutyl) styrene, 3-isopropenyl-xcex1, xcex1-dimethylbenzyl isocyanate, and halogenated styrenes.
Suitable acrylamide based monomers are, for example, acrylamide, N,N-dimethylacrylamide, N-octyl acrylamide, N-methylol acrylamide, dimethylaminoethylacrylate, etc. Suitable cyclic monomers are, for example, vinyl pyrrolidone, vinyl imidazolidone, vinyl pyridine, etc. Suitable sulfonated monomers are, for example, 2-acrylamido-2-methyl propane sulfonic acid, sodium methallyl sufonate, sodium vinyl sulfonate, sulfonated sytrene, etc. Suitable vinyl amide monomers are, for example, N-vinyl formamide, N-vinyl acetamide, etc.
In a preferred embodiment of the invention, the ethylenically unsaturated monomer is an alkyl acrylate monomer having the structure: 
wherein R1 is hydrogen or methyl and R2 is an alkyl group having from 1 to 10 carbon atoms. The alkyl groups in the alkyl acrylate monomers can be straight chained or branched. The ethylenically unsaturated monomer is preferably selected from methyl methacrylate, butyl acrylate, vinyl acetate, 2-ethylhexyl acrylate, and combinations thereof.
Optionally, an ionic monomer may be used to prepare the aqueous emulsion polymer in addition to the ethylenically unsaturated monomer in order to increase mechanical stability of the latex. Suitable ionic monomers include, for example, xcex1,xcex2-ethylenically unsaturated C3-C8 monocarboxylic acids, xcex1,xcex2-ethylenically unsaturated C4-C8 dicarboxylic acids, including the anhydrides thereof, and the C4-C8 alkyl half esters of the xcex1,xcex2-ethylenically unsaturated C4-C8 dicarboxylic acids. Preferred ionic monomers are acrylamido methyl propane, sulfonic acid, styrene sulfonate, sodium vinyl sulfonate, acrylic acid, methacrylic acid, and the C4-C8 alkyl half esters of maleic acid, maleic anhydride, fumaric acid, and itaconic acid. Most preferably, the ionic monomer is acrylic acid or methacrylic acid. The ionic monomer may be present in an amount of from about 0.01 to about 10 pphm, preferably from about 1 to about 5 pphm. Most preferably, the ionic monomer is present in an amount of from about 1.5 to about 3 pphm. A combination of ionic monomers may also be used.
The aqueous emulsion polymer is prepared with one or more surfactants or emulsifiers such as anionic and/or nonionic surfactants. The type and amount of surfactants are known in the art. Anionic surfactants include, for example, from C8 to C12 alkylbenzenesulfonates, from C12 to C16 alkanesulfonates, from C12 to C16 alkylsulfates, from C12 to C16 alkylsulfosuccinates or from C12 to C16 sulfated ethoxylated alkanols. Nonionic surfactants include, for example, from C6 to C12 alkylphenol ethoxylates, from C12 to C20 alkanol alkoxylates, and block copolymers of ethylene oxide and propylene oxide. The nonionic surfactants also include C4 to C18 alkyl glucosides as well as the alkoxylated products obtainable therefrom by alkoxylation, particularly those obtainable by reaction of alkyl glucosides with ethylene oxide. A combinations of surfactants may be used in preparing the aqueous emulsion polymer of the invention.
Water-soluble or water-dispersible polymerizable surfactants may also be used alone or in combination with nonpolymerizable surfactant(s) to prepare the aqueous emulsion polymer of the invention. A preferred polymerizable surfactant for preparing the aqueous emulsion polymer is an allyl amine salt of alkyl benzene sulfonate denoted Structure I: 
In Structure I, R3 is an alkyl group having 1 to 20 carbon atoms, preferably 10 to 18 carbon atoms; and X+ is selected from NH3+, NH2R6 or NR6R7 wherein R6 and R7 are independently C1-C4 alkyl or hydroxyalkyl groups. Most preferably, the allyl amine salt of alkyl benzene sulfonate is allyl amine salt of dodecylbenzene sulfonate.
Another preferred polymerizable surfactant is an allyl amine salt of alkyl ether sulfate denoted Structure II:

In Structure II, R4 is an alkyl group having 1 to 20 carbon atoms, preferably 10 to 18 carbon atoms; n is an integer from 2 to 15, and X+ is selected from NH3+, NH2R8 or NR8R7 wherein R6 and R7 are independently C1-C4 alkyl or hydroxyalkyl groups. Most preferably, the allyl amine salt of alkyl ether sulfate is allyl amine salt of laureth sulfate.
Another preferred polymerizable surfactant is an allyl amine salt of a phosphate ester denoted Structure III: 
In Structure III, R5 is an alkyl group having 1 to 20 carbon atoms, preferably 10 to 18 carbon atoms; n is an integer from 2 to 15, and X+ is selected from NH3+, NH2R6 or NR6R7 wherein R6 and R7 are independently C1-C4 alkyl or hydroxyalkyl groups. Most preferably, the allyl amine salt of a phosphate ester is allyl amine salt of nonyl phenol ethoxylate (9 moles EO) phosphate ester. Preferred polymerizable surfactants are available under the trademarks POLYSTEP AU1, POLYSTEP AU7 and POLYSTEP AU9 from Stepan Company.
The aqueous emulsion polymer is prepared using free radical emulsion polymerization techniques which are known in the art. The aqueous emulsion polymer may be prepared by emulsion polymerization methods which are known in the art and include batch or continuous monomer addition or incremental monomer addition processes. As used herein, xe2x80x9cbatchxe2x80x9d refers to a process whereby the entire amount of monomer is added in a single charge. As used herein, xe2x80x9ccontinuous monomer additionxe2x80x9d and xe2x80x9cincremental monomer additionxe2x80x9d refer to a process wherein optionally a minor portion of the monomer(s) is initially charged in the reactor and the remainder of the monomer(s) is then added gradually over the course of the reaction. The entire amount of the aqueous medium with polymerization additives can be present in the polymerization vessels before introduction of the monomer(s), or alternatively a portion of it can be added continuously or incrementally during the course of the polymerization.
Although the solids content and viscosity of the emulsion can vary, typical total solids content which is defined as the nonvolatile components of the emulsion is preferably in the range of from about 40 to about 70 weight percent, more preferably from about 50 to about 60 weight percent, based on the total weight of the emulsion.
The addition of an acidic, metal salt curing agent to the aqueous emulsion product after polymerization may be desired in order to accelerate the cure of the formulated adhesive. The preferred curing agents for use herein include acidic, metal salts selected from chromic nitrate, chromic perchlorate, aluminum nitrate, aluminum chloride, and para-toluene sulfonic acid. The amount of acidic metal salt curing agent which is added will depend on the rate of cure which is desired in the final product, however, a preferred range is from about 0.05 to about 4 weight percent, based on the total weight of the adhesive composition. A preferred acidic metal salt curing agent is zinc oxide.
The pressure sensitive adhesive compositions of the present invention may additionally contain other additives which include pigments such as titanium oxide, extenders such as flour, i.e., walnut shell flour, dispersing agents, defoaming agents, anti-freezing agents, preservatives, surfactants, sequestering agents, coalescing agents, defoaming agents, humectants, thickeners, defoamers, colorants, waxes, bactericides, fungicides, and fillers such as cellulose or glass fibers, clay, kaolin, talc, calcium carbonate and wood meal, and odor-modifying agents.
In preparing the pressure sensitive adhesive compositions of this invention, the aqueous polymer is mixed with the additive(s). The additive(s) may be added during the polymerization, after the polymerization and prior to the addition of the curing agent, or with the addition of the curing agent.
The pressure sensitive adhesive compositions may be applied to a wide variety of materials such as, for example, wood, cement, concrete, nonwoven or woven fabrics, aluminum or other metals, glass, ceramics, glazed or unglazed, tiles, polyvinyl chloride and polyethylene terephthalate and other plastics, plaster, stucco, roofing substrates such as asphaltic coatings, roofing felts, synthetic polymer membranes, and foamed polyurethane insulation. In addition, the adhesive compositions may be applied to previously painted, primed, undercoated, worn, or weathered substrates.
The pressure sensitive adhesives of the invention may be used in all types of tapes, labels, decals, packaging, electrical insulation, and surgical bandaging. The adhesives function to keep a backing or carrier surface to which the adhesive is applied in contact with a substrate to provide the substrate with resistance to corrosion (e.g., pipe wrapping tape), protection (e.g., surgical dressing), a mounting surface (e.g., double-faced tape), a closure or connection (e.g., diaper tape, splicing tape), or identification (e.g., labels, decals, stenciling).